Algol Consists of a 3

Multiple Choice

Algol consists of a 3.7 MSun main- sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?

A) The two stars should be the same age, so we'd expect the subgiant to be more massive than the main- sequence star.
B) A star with a mass of 3.7 MSun is too big to be a main sequence star.
C) It doesn't make sense to find a subgiant in a binary star system.
D) The two stars in a binary system should both be at the same stage of life; that is, they should either both be main sequence stars or both be subgiants.